Conventional plasma display devices are filled with rare gases (e.g., helium, neon, argon, xenon, and krypton) or mixtures of rare gases, which are excited by a high voltage electrical current to emit ultraviolet radiation in the VUV range below 200 nm in wavelength. This emitted VUV radiation is then used a primary source of excitation to excite various blue, green, and red emitting phosphors. The plasma display panels (PDP) are made with a back carrier plate, a transparent front plate, and a ribbed structure, which divides the space between the front and back plates into cells. The plasma display panels also contain a sophisticated electrode array, which can address and excite each discrete plasma cell individually. Each cell contains a small amount of the rare gas mixture and a small quantity of phosphor, which emits only one of the three colors. Cells containing phosphors which emit with each of the three distinct colors, red, green, and blue, are statistically distributed on the back plate within the panel in much the same fashion as the different colored pixels are distributed in other types of display panels, such as CRT displays. Like plasma display panels, VUV-excited lamps also contain rare gases or mixtures of rare gases and similar phosphors. The excitation-emission principles are similar to display panels except that blends of blue, green, and red phosphors are broadly coated on the inside of a lamp and emit with an intent to generate an overall white color instead of the three separate colors which are emitted by the various discrete plasma cells.
The most commonly used VUV excitation energy comes from xenon or xenon-helium plasmas, which emit in the region 147 nm to 173 nm. The exact emission spectra depends on the Xe concentration and overall gas composition. Under high voltage excitation, Xe-based plasmas typically have a Xe emission line at 147 nm and a Xe excimer band emission around 173 nm. This is very different from the primary 254 nm excitation radiation produced by the low-pressure, mercury vapor discharge of conventional fluorescent lamps. As a result, phosphors used in VUV-excited applications have new requirements imposed on them by the higher excitation energies as compared to conventional short-wave ultraviolet fluorescent applications.
In general, the phosphors used in VUV-excited devices exhibit some undesirable properties. However, the most problematic is the phosphor commonly used as the blue emitter, europium-activated barium magnesium aluminate (BAM), Ba1-xEuxMgAl10O17 (0.01<x<0.20). This phosphor is known to degrade in both brightness and color during the manufacturing process due to elevated temperatures and humidity. During manufacture of PDP panels, a thin MgO layer is applied for the purpose of protecting the transparent front plate and dielectric layer. MgO is quite hygroscopic and the high humidity conditions found during manufacture of the panels arise from water that is dissociated from the MgO layer during bakeout. This water is believed to be instrumental in the degradation in color causing a shift in the color point towards the green region. This phosphor also degrades in both brightness and color after extended exposure to the high intensity Xe plasma and VUV photon flux. The degradation mechanisms of BAM are the subject of much study and are thought to involve such changes as oxidation of Eu2+ to Eu3+, modifications in the actual structure of the aluminate phosphor lattice, and movement of the Eu2+ activator ions between different sites within the lattice. The actual life of a commercial plasma display panel is dramatically shortened due to the shift in the color point and reduction in intensity of the blue phosphor component, which leads to an undesirable yellow shift in the overall panel color. One highly relevant measure of this degradation is the ratio of the intensity (I) to the CIE y color point which can be calculated as a percentage. Both the intensity decrease and the increase in CIE y color coordinate (green shift) result in a reduction of the I/y ratio.
In recent years, a number of different approaches have been attempted in order to improve the maintenance of blue-emitting VUV-excited BAM phosphors. These approaches include sol-gel coating of wide bandgap metal oxides onto BAM phosphor, U.S. Patent Publication No. 2002/0039665; thermal treatments of aluminate phosphors mixed with ammonium fluorides, U.S. Pat. No. 6,242,043; and solution based catena-polyphosphate coatings of BAM phosphor, U.S. Pat. No. 5,998,047. Substitutional variations of the BAM stoichiometry have also been attempted in order to improve the maintenance of BAM such as substitution of alkali metals, alkaline earth metals, or zinc for one or more of the metallic components in BAM, U.S. Patent Publication No. 2002/0190240 A1. To date, none of these approaches have been entirely successful.
In addition, new phosphor compositions have been investigated which exhibit improved maintenance relative to commercial BAM phosphors such as (La1-x-y-zTmxLiySrz)PO4, U.S. Pat. No. 5,989,454; Ba1-aEuaMgAl6O11, U.S. Pat. No. 6,527,978; CaMgSi2O6:Eu2+; and CaAl2O4:Eu2+. A solid solution phase of BAM-barium hexa-aluminate (0.82BaO.6Al2O3) exhibits improved color stability and maintenance but has an undesirable color point. New mixtures or blends of blue VUV phosphors have also been disclosed with improved maintenance characteristics such as (La1-x-y-zTmxLiyAEz)PO4 mixed with BAM, Eu2+-activated barium magnesium lanthanum aluminate, Eu2+-activated alkaline earth chloroapatite, or Eu2+-activated calcium chloroborate phosphors, U.S. Pat. No. 6,187,225; and BAM or SCAP (Ba,Sr,Ca)5(PO4)3Cl:Eu mixed with a wide number of UV-C light emitting phosphors, U.S. Patent Publication No. 2001/0033133 A1.
Although many of these phosphors or phosphor complexes exhibit improvements in color and intensity stability, none have yet proven to be viable alternatives. Thus, there is still a need for improved blue-emitting VUV-excited phosphors. In particular, the following properties would be desirable: a deeper blue color, improved color stability during panel manufacture, improved lifetime during panel operation, and a high relative percent maintenance of the I/y ratio after accelerated thermal, humidity, Xe plasma, and high intensity VUV photon flux testing.